Developing an efficient and reliable source of EUV (Extreme Ultraviolet) radiation for semiconductor lithography is an active area of research. In one approach, a laser source, such as a CO2 laser, irradiates tin (Sn) droplets so that each droplet is heated into a plasma state. Radiation generated by the tin plasma includes radiation at a wavelength of about 13.5 nm. Radiation at other wavelengths may be filtered out, thereby providing an EUV source so that various lithographic steps may be carried out at the 13.5 nm wavelength. Such a source is referred to as an LPP (Laser Produced Plasma) EUV radiation source.
FIG. 1 is a simplified drawing illustrating some of the components of an LPP EUV radiation source 100. A droplet generator 102 provides drops of molten tin that are irradiated by a pre-pulse of radiation, indicated pictorially by the tin droplet 104 within the pre-pulse 106. It is to be understood that embodiments are not limited to tin as the irradiated material. The pre-pulse 106 shapes and expands a tin droplet into a target so that the target may better absorb the energy of a main pulse of radiation, where the main pulse heats the tin to a plasma state, or further converts the tin to a plasma state if the pre-pulse 106 is sufficient to heat some or all of the tin to a plasma state.
For example, the tin droplet 104 after being shaped by the pre-pulse 106 is represented by the tin target 108, shown as a flattened disc oblique with respect to the z axis of the reference coordinate system 110. The tin target 108 is shown within the main pulse 112 to pictorially represent irradiation of the tin target 108. A collector 114 focuses the EUV radiation generated by the tin plasma to some intermediate focus point at which the EUV radiation is provided to a lithography tool (not shown).
In the particular example of FIG. 1, the pre-pulse 106 is generated by a laser oscillator, referred to as the pre-pulse laser 116. The output of the pre-pulse laser 116 is provided to one or more power amplifiers 118 by way of one or more mirrors 120 and one or more dichroic splitters 122. The main pulse 112 is generated by a laser oscillator, referred to as a main pulse laser 124. The output of the main pulse laser 124 is also provided to the one or more power amplifiers 118.
In the particular example of FIG. 1, the wavelength of the pre-pulse laser 116 is different from that of the main pulse laser 124, where the one or more dichroic splitters 122 allow for the proper feeding of the outputs of the pre-pulse laser 116 and the main pulse laser 124 to the one or more power amplifiers 118. The pre-pulse 106 and the main pulse 112 are directed through an opening in the collector 114 to, respectively, the tin droplet 104 and the tin target 108. The obliqueness of the tin target 108 with respect to the z-axis helps to mitigate reflection of the main pulse 112 off of the tin target 108 and back through the opening of the collector 114 to the one or more power amplifiers 118. A portion of the output of the power amplifiers 118 is reflected off of the splitter 115 and to the power meter 117 so that the output of the power amplifiers 118 may be measured.
To mitigate the absorption of EUV radiation generated by the tin plasma, the irradiation of the tin droplets takes place in a chamber 119 held at low pressure, where the low pressure may be referred to as a vacuum. Accordingly, some modules for providing the pre-pulse 106 and the main pulse 112 to the tin, such as the final focus module 121, are utilized within a vacuum and therefore are shown within the chamber 119. Other modules may be utilized at ordinary atmospheric pressure. A window 123 in the chamber 119 serves as an optical interface between the chamber 119 and other modules at ordinary atmospheric pressure, so that the pre-pulse 106 and the main pulse 112 provided to the final focus module 121 may be generated by modules operating at ordinary atmospheric pressure.
For ease of illustration, the modules outside the chamber 119 for focusing and steering the pre-pulse 106 and the main pulse 112 are lumped together into the beam steer and focus module 126. Other structures and arrangements of one or more laser oscillators may be utilized to provide the pre-pulse 106 and the main pulse 112 to the tin. For example, a single seed laser may be used in the generation of the pre-pulse 106 and the main pulse 112. Not all components important for the generation of EUV radiation are illustrated. For example, not shown in FIG. 1 are pumps to evacuate gas from the chamber 119, control units for the laser oscillators, and modules for mitigating the effects of debris from the irradiated tin.
The beam steer and focus module 126 includes the dichroic splitter module 128, shown in expanded form by way of arrow 129. The label “MP” refers to main pulse and the label “PP” refers to pre-pulse. The two dichroic splitters 130 and 132 allow the pre-pulse 106 to pass through the dichroic splitter module 128 toward the tin droplet 104. The combination of the dichroic splitter 130, the mirror 134, the mirror 136, and the dichroic splitter 132 reflects the main pulse 112 toward the tin target 108. The relative orientation of the mirror 136 to the dichroic splitter 132 steers the main pulse 112 in a direction different from that of the pre-pulse 106. One or both of the mirrors 134 and 136 may be curved so that the focus of the main pulse 112 is in a different focal plane than that of the pre-pulse 106 to take into account the displacement of the tin target 108 relative to the tin droplet 104.
The beam steer and focus module 126 includes the final focus metrology module 131. Fractional portions of the pre-pulse 106 and the main pulse 112 are reflected off of the window 123 and directed by way of the mirror 133 to the final focus metrology module 131 so that various metrology and diagnostic functions of the pre-pulse 106 and the main pulse 112 may be carried out. Included in the final focus metrology module 131 is the wavefront sensor 135 to measure the intensity and phase at various points in a wavefront of the main pulse 112. The beam steer and focus module 126 may include other optics, but for simplicity only the dichroic splitter module 128, the final focus metrology module 131, and the mirror 133 are illustrated.
FIG. 2 expands upon the drawing of the tin droplet 104 and the tin target 108 of FIG. 1. It may be desirable to irradiate the tin droplet 104 and the tin target 108 so that efficient use is made of the irradiating power, and so that the temperature of the resulting plasma is close to some specified optimum temperature so as to maintain a desirable conversion efficiency. Conversion efficiency may be defined as the ratio of EUV radiation energy generated by the tin plasma to the energy required to generate the main pulse 112.
It is expected that to maintain conversion efficiency at some specified level, the main pulse 112 should irradiate the tin target 108 at some specified constant (or nearly constant) irradiance. Consequently, it is expected that the beam diameter of the main pulse 112 at the position of the tin droplet 108 should be commensurate with the surface area of the tin droplet 108 presented to the main pulse 112, where the irradiance is at some specified level. (The beam diameter at some position may also be referred to as the diameter of the caustic at that position, or simply the caustic.) Accordingly, the pre-pulse focal plane 202, which may be taken as the plane intersecting the beam waist of the pre-pulse 106, may not coincide with the position at which the tin droplet 104 is irradiated. Similarly, the main pulse focal plane 204, which may be taken as the plane intersecting the beam waist of the main pulse 112, may not coincide with the position at which the tin target 108 is irradiated.
As illustrated in FIG. 2, there is a displacement Δx along the x-axis from where the tin droplet 104 is irradiated to where the tin target 108 is irradiated. The displacement Δx may vary over time, and may depend upon the size of the tin droplet 104 and its speed of descent along the x-axis. The tin droplet 104 may be formed out of multiple smaller droplets emitted from the droplet generator 102, where one or more of these smaller droplets coalesce into the tin droplet 104. There is also a displacement Δz along the z-axis from where the tin droplet 104 is irradiated to where the tin target 108 is irradiated. The displacement Δz is in part due to irradiating and heating the tin droplet 104, and may depend upon the size of the tin droplet 104 and its speed of descent. Consequently, the displacement Δz may vary over time.
FIG. 2 is idealized in that the beams are illustrated as having a finite profile perfectly matched to the size of the tin droplet 104 or the tin target 108. In practice, the beams have a profile for which the majority of the irradiance is intercepted by the tin droplet 104 or the tin target 108.
The EUV radiation power resulting from the tin plasma formed by irradiating the tin target 108 may depend among many variables, some of which may include the sizes of the tin droplet 104 and the tin target 108, and the rate of tin droplet generation. Accordingly, the displacements Δx and Δz may depend upon the power operating node of the LPP EUV radiation source 100, so that the positions of the pre-pulse focal plane 202 and the main pulse focal plane 204 may need to be adjusted depending upon the power operating node. However, adjusting the optics to re-position the focal planes may be expensive and time consuming for presently available LPP EUV sources.